The use of high temperature superconducting ceramic articles and nonsuperconducting ceramic articles requires reproducible production of the articles from ceramic powders with high densities, high purities, good homogeneity, and fine grain size. Processing of ceramic powders for production of such ceramic articles indicates that these characteristics can be achieved by starting with submicron ceramic powder composed of equiaxed or substantially symmetrical particles with a homogenous chemical composition and a narrow but not necessarily monodispersed size distribution.
A common method for producing superconducting ceramic powders involves grinding ceramic powders produced by solid state reactions. The grinding provides particles that are greater than one micron in size, are not equiaxed, have a broad particle size distribution, and are often contaminated by the grinding media. Techniques such as sol-gel, precipitation, and freeze-drying have been developed to overcome some of these undesirable features of ceramic particles produced by grinding; however, most of these alternative techniques for producing ceramic particles cannot directly produce superconductive ceramic particles. The sol-gel and precipitation methods rely upon gelation or precipitation, both of which are difficult phenomena to predict and control. For example, when mixed metal cation ceramics are to be produced, gelation or precipitation of the separate components or phases should occur somewhat simultaneously and at about the same rate so that the mixing of the different cations is homogenous. The segregation of the cations is particularly troublesome when the gelation or precipitation is carried out in a batch, such as a beaker, because the distance the cations can become segregated is large compared to the size of the final powders.
Another attempt at producing submicron superconducting ceramic powder as well as nonsuperconductive ceramic powder composed of equiaxed particles with homogenous chemical composition and a narrow size distribution involves passing a ceramic precursor solution through an aerosol generator to convert the solution into a plurality of fine droplets. The droplets are then carried through a furnace which supplies substantially all the thermal energy that causes the solvent in the solution to evaporate and the ceramic precursor materials to decompose to form ceramic particles. Because the furnaces typically operate at high temperatures on the order of 1000.degree. C., the external energy requirements are high, which leads to high energy costs of operation.
It has been reported that nitrogen containing compounds such as urea and glycine can be used as a fuel to provide combustion on a batchwise scale for the conversion of pastes or solutions of metal nitrates and glycine or urea to ceramics. The glycine is reported to complex with the metal cations, allowing the solution to be thickened to a honey-like consistency before the solvent is evaporated. Upon evaporation of the solvent, a viscous foam is formed and eventually ignites. The urea/metal nitrate solution is reported to form a polymeric gel upon combustion. The polymeric gel is converted into a foam by the gases produced by the combustion. The production of intermediates with a honey-like consistency or a polymeric gel prior to or during combustion is dangerous because buildup of these less than completely reacted intermediates on process equipment can create the potential for an uncontrollable explosive reaction.
Although the techniques described above have shown some success in producing submicron superconductive and nonsuperconductive ceramic powders composed of equiaxed particles with homogenous chemical composition and/or narrow size distribution, such prior techniques have very high and costly external energy requirements. For economic reasons, in the increasingly competitive ceramic industry, it would be advantageous to produce ceramic powders having the characteristics described above by a process that requires less external energy than previously available processes.